08b_volcano_09_post - Types of volcanic features • Flood...

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Unformatted text preview: Types of volcanic features • Flood basalts: regional lava flows that come from fissures (long, linear cracks in the earth's surface); thick accumulations build lava plateaus Fig. 9.9 Lecture 8: 1 Types of volcanic features Krafla fissure eruption, 1977 Northern Iceland Lecture 8: 2 Types of volcanic features Types of volcanic features Krafla splatter cones, northern Iceland Photo: R.W. Schlische Krafla fissure eruption, , 1980 Northern Iceland Splatter cones along fissure eruption Lecture 8: 3 Lecture 8: 4 Types of volcanic features • Flood basalts / fissure eruptions Laki crater row, southern Iceland Photo: Y. Arthus-Bertrand Types of volcanic features • Flood basalts / fissure eruptions Fissure on Kilauea’s western rift Photo: M. Levy Columbia River basalt and basalt of Snake River Plain are flood basalts. Lecture 8: 5 Lecture 8: 6 Types of volcanic features • Ancient flood basalts of Columbia River region Products of eruptions • Gases: CO2, H2O, SO2, H2S, HCl, N2 Photo: P.L. Kresan Lecture 8: 7 Fig. 9.18 Photo: P.L. Kresan Lecture 8: 8 Products of eruptions Products of eruptions • Pyroclastic (fragmental) material, e.g., ash, bombs • Pyroclastic (fragmental) material, e.g., ash, bombs Volcanic bomb ejected from Asama volcano, Japan. Photo: Science Source Paricutin cinder cone eruption in 1944, Mexico Photo: E. Tad Nichols Lecture 8: 9 Lecture 8: 10 Products of eruptions Products of eruptions Lava fountain and lava “rivers” rivers” spewing from Pu’u Pu’ ‘O’o, Kilauea Lava “falls”, falls” Kilauea Photo: USGS Photo: USGS Lecture 8: 11 Lecture 8: 12 Types of basaltic lava Products of eruptions • Pahoehoe lava: billowy and ropelike (hotter) “Fossil” lava “falls”, Kilauea Fossil” falls” Active pahoehoe flow, Kilauea Photo: R.W. Schlische Lecture 8: 13 Types of basaltic lava Photo: USGS Lecture 8: 14 Types of basaltic lava • Pillow lava: spheroidal masses produced by underwater eruptions • Aa: rough surface composed of blocks and fragments (cooler) Active aa flow, Kilauea Fig. 6.23 Photo: USGS Lecture 8: 15 Photo: W.H.O.I. Lecture 8: 16 Plate tectonics / viscosity Viscosity of lava • Viscosity: resistance to flow • Viscosity increases as silica content increases Continental rifting / continental hotspots • Basalt -- low viscosity; flows easily Convergent boundaries • Rhyolite -- high viscosity; does not flow easily Divergent boundaries & oceanic hot spots Lecture 8: 17 Lecture 8: 18 Plate tectonics / viscosity Continental rifting / continental hotspots Convergent boundaries Volcanic eruptions • George Julius Poulett Scrope (17971896, England): “…the main agent in all these stupendous phenomena consists unquestionably in the expansive forces of some elastic aëriform fluid struggling to escape from the interior of a subterranean body of lava.”—Considerations on Volcanoes, 1825. Divergent boundaries & oceanic hot spots Fig. 9.2 Lecture 8: 19 Lecture 8: 20 Volcanic eruptions Volcanic eruptions • Magma contains dissolved gases • Importance of gas is indicated by pyroclastic rocks • Rising magma --> pressure reduction • --> gas release • --> eruption • Soda-bottle & beer-pouring analogy Pumice, scoria--volcanic rock containing numerous cavities filled with gas Ash--shards of volcanic glass formed in explosion Lecture 8: 21 Lecture 8: 22 Volcanic eruptions Volcanic eruptions • Basaltic lava: gas escapes readily-->quiet eruptions Fig. 9.2 • Viscous lavas: gas escape is impeded --> buildup in gas pressure-->explosive eruptions Fig. 9.5 1991 Eruption of Mt. Unzen, Japan. Photo: AP Lecture 8: 23 Kilauea Volcano, Hawaii. Photos: USGS Lecture 8: 24 Types of volcanoes Review Questions 8-1. A. True / B. False: Volcanism was responsible for producing the Earth's atmosphere. Fig. 9.12 Largest; gentlest slopes; very fluid lava 8-2. A. True / B. False: A caldera is larger than a crater. 8-3. A. True / B. False: Viscosity increases as the silica content of lavas increases. 8-4. Which type of lava has the lowest viscosity? A. andesitic B. basaltic C. rhyolitic Viscous lava & pyroclastics 8-5. A. True / B. False: Explosive volcanic eruptions are associated with highviscosity lava because gases cannot readily escape, leading to a build-up in pressure and eventual explosion. Smallest; steepest slopes; pyroclastics 8-6. Which type of lava has a relatively smooth, ropy or billowy surface? A. aa B. pahoehoe C. pillow 8-7. Which type of lava has a very rough surface? A. aa B. pahoehoe C. pillow Lecture 8: 25 Review Questions 8-8. Which type of lava forms under water? A. aa B. pahoehoe C. pillow 8-9. A. True / B. False: Composite volcanoes are commonly found at hot spots. 8-10. A. True / B. False: Eruptions associated with composite volcanoes tend to be much less destructive than eruptions associated with shield volcanoes. 8-11. A. True / B. False: Rhyolitic magmas are associated with lava domes. 8-12. A. True / B. False: Shield volcanoes have the gentlest slopes because the very fluid basaltic lavas can flow large distances before cooling. 8-13. Of the three primary forms of subaerial volcanoes, ____________ are the largest in area. A. stratovolcanoes B. cinder cones C. shield volcanoes 8-14. Of the three primary forms of subaerial volcanoes, ____________ are the smallest in area. A. stratovolcanoes B. cinder cones C. shield volcanoes Lecture 8: 27 Lecture 8: 26 Review Questions 8-15. Explosive or voluminous eruptions may cause the volcano to collapse on the floor of the (now empty) magma chamber, producing a broad depression termed a ____________. A. crater B. lahar C. caldera D. fissure 8-16. The smoky cloud that rises from the vent of an actively erupting volcano is composed of ____________. A. smoke from wildfires inside the volcanic chimney B. smoke from wildfires on the outer slopes of the volcano that become funneled into the crater C. fine volcaniclastic debris (ash) suspended in the air D. a continuous fountain of dark, basaltic lava 8-17. Rhyolitic lavas ____________. A. do not flow as far from the vent as basaltic lavas do B. cool much more slowly than basaltic lavas do C. are associated with volcanoes that almost never emit pyroclastic debris 8-18. Pillow lavas are associated with ____________. A. continental rhyolitic eruptions B. continental basaltic eruptions C. submarine rhyolitic eruptions D. submarine basaltic eruptions Lecture 8: 28 ...
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